1. Field of Invention
The present invention relates to an electromagnetic motor that generates thrust from a magnetic flux using Lorenz's forces generated by flowing current in a coil. In particular, this electromagnetic motor is especially suitable for use with a stage used in an Electron Beam (EB) exposure device or in a projection exposure device used to manufacture, for example, integrated circuits (ICs), liquid crystal displays (LCDs) and other thin film devices.
2. Description of Related Art
A conventional linear type electromagnetic force motor is explained referring FIGS. 9A and 9B. FIG. 9A is a plan view of a linear type electromagnetic motor. FIG. 9B is a cross sectional view of FIG. 9A, which is cut along the line B--B. The electromagnetic motor is schematically composed of a pole unit 400 and a thrust generating unit 300. The pole unit 400 is composed of, for example, low carbon steel as a magnetic member and a rectangular parallelepiped shaped yoke 208 that extends in the X direction in the figure. A plurality of permanent magnets 204 and 210 that are alternately aligned (i.e., the N and S poles of adjacent magnets are alternately arranged) with a predetermined pitch P in the X direction are provided on the +Y direction side surface of the yoke 208. The permanent magnets 204 and 210 have magnetic axes that approximately match in the Y direction. The S poles of the permanent magnets 204 are polarized on the surface facing in the +Y direction, and the N poles are polarized on the surface facing in the -Y direction. On the other hand, the N poles of the permanent magnets 210 are polarized on the surface facing in the +Y direction, and the S poles are polarized on the surface facing in the -Y direction. Additionally, both magnetic pole ends of each of the permanent magnets 204 and 210 have a width of P/2 in the X direction.
The thrust generating unit 300 sandwiches the pole unit 400 from the +Y direction and the -Y direction, and has two cores 200 and 216 that are composed of, for example, low carbon steel as a magnetic member. An armature coil 202, which opposes the end surfaces of the plurality of permanent magnets 204 and 210 with a specified spacing, is provided on the +Y direction surface side of the yoke 208. The armature coil 202 has an opening in the X-Z surface (i.e, the surface contained in the XZ plane), and the coil is wound so that the aforementioned opening extends in the Y direction. The two surfaces of the armature coil 202 that are opposed to the permanent magnets 204 and 210 have a width of P/2 in the X direction, respectively, and have a longer width than the length of the permanent magnets 204 and 210 in the Z direction as shown in FIG. 9B. Additionally, the distance between centers of the two surfaces of the armature coil 202 equals the pitch P in the X direction.
Regarding the yoke 208 of the pole unit 400 and the core 216 that opposes it from the -Y direction, an armature coil 214 is provided on core 216 and is opposed to the end surfaces of a plurality of permanent magnets 206 and 212 that are provided on the -Y direction side surface of the yoke 208 with a predetermined spacing therebetween. The armature coil 214 has an opening in its X-Z surface, and the coil is wound so that the aforementioned opening extends in the Y direction. The two surfaces of the armature coil 214 that are opposed to the end surfaces of the permanent magnets 206 and 212 have a width of P/2 in the X direction, and have a longer width than the length of the permanent magnets 206 and 212 in the Z direction. Additionally, the distance between the centers of these two surfaces is equal to the length of the pitch P in the X direction.
A specified current is supplied from a power device, which is not shown in the figure, to the armature coils 202 and 214. Additionally, the pole unit 400 is supported so as to be movable in the X direction relative to the thrust generating unit 300 by a supporting mechanism, which is not shown in the figure.
In the electromagnetic motor having the above-mentioned structure, when the thrust generating unit 300 and the pole unit 400 are in the relative position shown in FIGS. 9A and 9B, a magnetic flux loop 218 of one cycle is formed as shown in the figure. The magnetic flux of this magnetic flux loop 218, for example, starting at the permanent magnet 204, comprises a closed magnetic path that goes through the yoke 208 of the magnetic body, the permanent magnet 206, passes through the surface of the armature coil 214 that extends in the Z direction, goes through the magnetic core 216 and passes through the other side surface of the armature coil 214 that extends in the Z direction, reaches the permanent magnet 212, passes through the yoke 208 and the permanent magnet 210, passes through a surface of the armature coil 202 that extends in the Z direction, passes through the magnetic core 200, and passes through the other side surface of the armature coil 202 that extends to the Z direction, and returns to the permanent magnet 204.
The current flows in the direction shown in FIG. 9A with respect to the armature coils 202 and 214 from a power device, not shown in the figure. In FIG. 9A, the ".cndot." symbol indicates that the current flows outward from the paper surface, and the "x" symbol indicates that current flows into the paper surface. Hereafter, the ".cndot." and "x" directions are called the axis directions of the coil.
As shown in the figure, by supplying current in the Z direction through the armature coils 202 and 214, a Lorentz's force is generated in the X direction in accordance with Fleming's left hand rule in the areas where magnetic flux passes out of the armature coils 202 and 214 and into the permanent magnets 204, 206, 210 and 212. When the thrust generating unit 300 is anchored at a specified position, the pole unit 400, which is movably held in the X direction by a supporting member (not shown in the figure) shifts in the +X direction by using the reaction of the Lorentz's force. By doing this, an electromagnetic motor can be structured with the thrust generating unit 300 as the stationary part and the pole unit 400 as the moving part. Further, a planar motor can be structured by developing this linear type electromagnetic motor in two-dimensions.
However, with the conventional electromagnetic motor explained above, the following problems occur.
(1) Problems with high thrust force and high efficiency
The permanent magnet surfaces of the magnets 204, 206, 210 and 212 that are used in the conventional electromagnetic motor are directly arranged so that magnetic flux passes parallel to the winding axes of the armature coils 202 and 214 of the thrust generating unit 300, as is clear from FIG. 9A. Accordingly, in order to improve the thrust, a permanent magnet having a high energy must be used, and accordingly the magnetic path cross-sectional area of the magnetic circuit has to be enlarged. However, with this method, there are problems in that the total cost of the permanent magnet is large and the efficiency becomes poor since the weight of the pole unit 400 when it is used as the moving part becomes large.
(2) Problems of preventing the failure of the magnets
In the conventional electromagnetic motor, there are problems in that the permanent magnets 204, 206, 210 and 212 are arranged in order to have a magnetic axis (magnetic direction) parallel to the winding axes of the armature coils 202 and 214. In this case, the magnetic field produced by excitation of the armature coils 202 and 214 (due to the corkscrew rule) also operates as a demagnetization field with respect to the permanent magnets 204, 206, 210 and 212. Therefore, the permanent magnets 204, 206, 210 and 212 become demagnetized or degaussed. Another problem is that the magnet surface burns due to the eddy currents generated in the permanent magnets 204, 206, 210 and 212.
(3) Problems of high thrust density
In a planar electromagnetic motor in which the conventional linear electromagnetic motor is two dimensionally developed, there are problems in that a large surface area is necessary since armature coils must be separately provided for the coil for X axis driving and the coil for Y axis driving. These drive coils for each axis are anchored and arranged on independent areas on the thrust generating unit, and the driving force per unit has to be small.
Another problem is that since the X axis driving force and the Y axis driving force operate in different positions, unnecessary rotation force is generated in the moving part, the control of which becomes difficult.